Global control system-plasma synchronization and naturally occurring edge localized modes in the ASDEX Upgrade tokamak

Global control system-plasma synchronization and naturally occurring edge localized modes in the ASDEX Upgrade tokamak

Global control system-plasma synchronization and naturally occurring edge localized modes in the ASDEX Upgrade tokamak 150 150 Mathew
CCFE-PR(17)48

Global control system-plasma synchronization and naturally occurring edge localized modes in the ASDEX Upgrade tokamak

The ITER tokamak needs to sustain a plasma in a regime of high energy confinement (H-mode) to exceed fusion breakeven where power output exceeds input. H-mode plasmas are typically unstable to edge localised modes (ELMs), in which plasma escapes and strikes the plasma facing components. Scaled up to ITER, the energy released by ELMs can cause critical damage and needs to be addressed to achieve sustainable breakeven. Proposed methods for ELM control include externally triggering smaller, more frequent ELMs by injecting pellets of frozen deuterium that modify the plasma edge, or by externally applying magnetic kicks by pulsing the current in toroidal magnetic field coils near the plasma boundary. Maintaining a steady state plasma requires active control and this control system includes these global field coils. The standard paradigm is that the control system acts on a relatively short timescale to restore the plasma steady state following an instability such as an ELM. We find that under certain conditions the plasma transitions into a state in which the control system current in these field coils continually oscillates and is synchronized with oscillations in the plasma edge position and several characteristic plasma parameters such as total MHD energy. These synchronous oscillations have a one-to-one correlation with the naturally occurring ELMs; the ELMs all occur when the control system coil current is around a specific phase. In this synchronous state, there is a continual non-linear feedback between the active control system and the global plasma dynamics that is intrinsic to the natural ELMing process. This supports the new paradigm that the nonlinear feedback between plasma and control system is an intrinsic part of the cyclic dynamics of naturally occurring ELMs for which there is evidence on JET. Real time knowledge of the control system signal phase indicates future times when ELM occurrence is more likely.

Collection:
Journals
Journal:
Plasma Physics and Controlled Fusion
Publisher:
IOP
Published date:
07/01/2017